It is desirable that low noise and a wide dynamic range are compatible as characteristics of an analog/digital conversion circuit (AD conversion circuit) to be used in a medical image diagnostic apparatus such as a CT (Computed Tomography) scanner. Specifically, a radiation transmitted through a low-density body part is very high, and it is said that the AD conversion circuit with a 120-dB dynamic range is required to convert analog signals detecting the radiation into digital signals. However, on the other hand, a radiation transmitted through a bone or a large object is very low, and it is required that noise of the AD conversion circuit is extremely low.
In order to solve these two problems, Patent Literature 1 and Patent Literature 2 disclose an analog/digital conversion circuit that integrates output of a detector that detected a radiation with an integral amplification circuit using an AD converter whose dynamic range is limited, an integration circuit, and a comparator and converts the output of the integral amplification circuit into digital signals by performing sampling with the AD converter.
The AD converter samples output of the integration circuit a plurality of times at a predetermined sampling period during a predetermined measurement period. When the comparator detects that output of the integral amplification circuit reaches a predetermined maximum voltage level, the output of the integral amplification circuit is reset to a predetermined minimum voltage level. This configures a circuit capable of converting a high-intensity radiation into digital signals with the AD converter whose dynamic range is limited. In Patent Literature 1, provided are an isolation circuit isolating the integral amplification circuit from the detector during a sampling event of the AD converter, an isolation circuit isolating the integral amplification circuit from a feedback capacitor when the integral amplification circuit is reset, and the like in order to reduce noise of the AD converter. Hence, disclosed is a technique capable of converting a low-intensity radiation into digital signals with low noise.